Peptide Glossary: Laboratory, Analytical and Regulatory Terms Defined

Last reviewed 9 August 2026

Every term below is one this site uses — in a compound library entry or a research article — or one the definitions below rest on. Where a term carries a legal or standards definition and that text is publicly available, the definition is given with its source rather than paraphrased: paraphrase is how a precise term quietly becomes an imprecise one. Where the governing text is not publicly available — the compendial monographs are behind a paywall — the entry says so rather than sourcing a figure by inference.

Two habits are worth carrying into it. Several of these words are read as though they graded quality — research grade, purity, bacteriostatic, certified. None does; each states a specification, a measurement or a container property. And a term naming a class — secretagogue, analogue, growth factor — describes a relationship between molecules, not what any molecule does.

Core concepts

• Peptide — the IUPAC-IUB Joint Commission on Biochemical Nomenclature defines one as “any compound produced by amide formation between a carboxyl group of one amino acid and an amino group of another”. That linkage is the peptide bond, and it is what hydrolyses when a chain breaks down in solution.
• Residue — the part of an amino acid remaining in the chain once the bond has formed. Chain length is counted in residues: a fifteen-residue peptide is one molecule of fifteen linked parts.
• Oligopeptide and polypeptide — the same recommendations state that peptides “with fewer than about 10–20 residues may also be called oligopeptides; those with more, polypeptides”. Convention, not measurement.
• Sequence — the ordered list of residues, written left to right from the amino terminus. Written form and chemical name run the same way, which is why glycine condensed with alanine is glycylalanine and not the reverse. See how sequences are notated.
• N-terminus and C-terminus — the ends carrying a free amino and a free carboxyl group. In three-letter symbolism a hyphen to the left denotes “removal of a hydrogen atom from the 2-amino group” and one to the right “removal of hydroxyl from the 1-carboxyl group”. Gly- and -Gly differ.
• Analogue — a molecule related to a named parent by a defined structural change. Two analogues of one parent can have separate literatures and separate regulatory positions.
• Fragment — a contiguous sub-sequence of a longer parent. The 2026 Prohibited List uses the term this way in naming “growth hormone fragments, e.g. AOD-9604 and hGH 176-191”. Evidence on a parent does not automatically describe a fragment of it, as the TB-500 entry sets out.
• Blend — a preparation holding more than one named compound in one container. What is in a blend sets out composition and the evidence consequence of combining them.

Compound classes

• Peptide hormone — a signalling molecule that is itself a peptide, and the organising term for section S2 of the World Anti-Doping Agency Prohibited List: Peptide hormones, growth factors, related substances, and mimetics.
• Secretagogue — a substance acting on a secretory pathway rather than on the receptor of the secreted molecule. The 2026 List groups “growth hormone secretagogues (GHS) and their mimetics” at S2.2.4, naming ibutamoren (MK-677) and ipamorelin among others. More sit in this class archive.
• GH-releasing peptide (GHRP) — listed separately from secretagogues in the same sub-section, naming examorelin under its alternative designation hexarelin, and GHRP-2 (pralmorelin) through GHRP-6.
• GHRH analogue — a molecule related to growth hormone-releasing hormone. S2.2.4 names CJC-1295, CJC-1293, sermorelin and tesamorelin as examples.
• Growth factor — S2.3 is headed Growth factors and growth factor modulators and names “Thymosin-β4 and its derivatives e.g. TB-500”. Two compounds sold together can therefore sit in different sections of one List, as this comparison examines.
• Mitochondrial-derived peptide — a peptide whose coding sequence lies in mitochondrial rather than nuclear DNA. Lee and colleagues (2015) reported a short open reading frame within the mitochondrial 12S rRNA encoding a sixteen-residue peptide, which they named MOTS-c. Three literatures grouped under this heading are kept separate.
• Metal complex — a peptide coordinated to a metal ion. GHK-Cu is a copper(II) complex of a tripeptide, and the complex rather than the free tripeptide is what most of its literature concerns.
• Diluent — a liquid used to take a lyophilised solid into solution. Not a research compound, and carrying no peptide purity specification; the two stocked here sit under reconstitution consumables.

Laboratory and handling terms

• Lyophilisation (freeze-drying) — removal of water by sublimation from the frozen state. The formulation literature handles dried and solution states as separate stability problems with separate design rules.
• Cake — the porous solid left in the container afterwards. Structure is a process outcome: Johnson and colleagues (2002) reported that 4% mannitol with 1% sucrose gave a cake of crystalline mannitol and amorphous sucrose, the crystalline phase permitting far warmer primary drying than sucrose alone would allow without collapse.
• Bulking agent — an excipient (any component other than the named compound) added to give the cake structure, mannitol being the usual example. A lyoprotectant, such as the sucrose above, is there to limit structural perturbation during freezing and drying.
• Reconstitution — taking a lyophilised solid into solution with a diluent. The resulting concentration is the mass of solid divided by the volume of diluent, in milligrams per millilitre. The arithmetic is set out here and computed by the calculator.
• Concentration — mass per unit volume, a property of the solution and the only quantity this site calculates: the volume is arithmetic and is ours to calculate, a target amount is not.
• Graduation — a marked division on a measuring barrel. A U-100 barrel divides one millilitre into a hundred units, so a unit is a volume, not an amount of substance. That conversion is an identity.
• Hygroscopic — taking up water from the surrounding atmosphere. Residual water is a determinant of solid-state stability, hence sealed containers and dry handling in every storage specification.

Analytical and quality terms

• HPLC area percent — the proportion of total detected peak area attributable to the main peak: a statement about what one detector saw under one set of conditions, not about how much material is in the container.
• Net peptide content — the proportion of the contents that is peptide rather than counter-ion, residual water and salts. Two numbers therefore circulate under the word purity, and a certificate should say which it reports.
• Counter-ion — an ion associated with a charged peptide. Roux and colleagues (2008) noted that synthesis and purification leave cationic peptides mainly as trifluoroacetate salts, that the trifluoroacetate is tightly bound and interferes with infrared characterisation, and reported complete removal from an octapeptide only by deprotonation and reprotonation.
• Average mass and monoisotopic mass — the average figure uses standard atomic weights, themselves abundance-weighted across naturally occurring isotopes; the monoisotopic figure uses the most abundant isotope of each element. The two diverge as a molecule grows; at peptide masses a spectrum is read against the monoisotopic figure, while the average figure is the one used for larger proteins. IUPAC standardises the vocabulary.
• Identity confirmation — a measured mass conforming to the mass computed from a stated sequence. It establishes that the material is what the label names, and nothing about purity or quantity.
• CAS Registry Number — a numeric identifier assigned to a substance record. It identifies; it does not qualify, and holding one implies nothing about regulatory status.
• Endotoxin — lipopolysaccharide from the cell wall of Gram-negative bacteria. The FDA guidance, now in its second edition of March 2026, sets out testing recommendations by reference to United States Pharmacopeia chapter <85>, Bacterial Endotoxins Test, USP chapter <161> and AAMI ST72:2002/R2010, which it states “describe the fundamental principles of the gel clot, photometric, and kinetic test methods”. Separate from sterility.
• Certificate of analysis — a document reporting specified tests on a named lot, a defined quantity produced in one operation. A certificate naming no lot is not traceable to the container in front of the reader. NovoVita does not publish certificates of analysis; its published position on purity is one sentence: “Third-party tested at greater than 99% purity.” The reasoning is set out separately.

Storage and documentation terms

• Hydrolysis — cleavage of a bond by water; one of the chemical routes catalogued in the stability literature, and faster in solution than in a dried solid. Aggregation, the association of molecules into larger species, is classed as physical rather than chemical instability; the two interact.
• Deamidation — non-enzymatic conversion of the side-chain amide of asparagine or glutamine. A sequence holding neither residue cannot deamidate, which is why degradation routes are read off the sequence rather than assumed. Oxidation is read the same way, principally at methionine and cysteine.
• Freeze–thaw — repeated cycling between frozen and liquid states, described in the literature as a stress in its own right, not merely time spent cold.
• Ambient excursion — a period at ambient temperature outside the specified storage condition, typically in transit. What the handling literature says about short excursions is a stability question, not a courier one.
• The 28-day convention — a figure attaching to a preserved multiple-entry container and its closure. It describes the vehicle, not anything dissolved in it, and has no application to an unpreserved single-use presentation. It is a compendial convention rather than a measurement, and the monograph text is not quoted here because it is not publicly available; the preservative it attaches to and the two diluents are covered separately.
• Bacteriostatic — restricting the proliferation of organisms, as distinct from bactericidal, which is killing them. Neither means sterilising; no preservative concentration makes a contaminated container clean.
• Last reviewed — the date a page was last checked against its sources. It moves only when content changed: a freshness stamp advancing on a no-op edit is a false statement on a page whose value is making none.

Regulatory terms

• Medicinal product — defined in regulation 2 of the Human Medicines Regulations 2012 in two limbs, quoted in MHRA Guidance Note 8 as “Any substance or combination of substances presented as having properties for treating or preventing disease in human beings” and “Any substance or combination of substances which may be used in, or administered to, human beings, either with a view to restoring, correcting or modifying physiological functions by exerting a pharmacological, immunological or metabolic action, or to making a medical diagnosis”. Falling under either limb is sufficient.
• By presentation — shorthand for the first limb. Status under it turns on how a product is presented, so a claim rather than a molecule is what engages it. The UK position is set out here.
• Disease — regulation 8 defines this as including “any injury, ailment or adverse condition, whether of body or mind”. That breadth is why the headings in the library and in these articles are model classes and document sections rather than conditions.
• Marketing authorisation — Guidance Note 8 states that, unless exempt, any medicinal product on the UK market must hold one, or a traditional herbal or homoeopathic registration, from the UK Licensing Authority.
• Research use only — a statement of the terms on which material is supplied. Not a regulatory classification, and it does not settle a product’s status, which the MHRA assesses case by case on all available evidence.
• Prohibited List — a mandatory International Standard of the World Anti-Doping Code, updated annually; the 2026 edition took effect on 1 January 2026. Sections S0 to S5 and M1 to M3 are prohibited at all times, S6 to S9 in-competition, P1 in particular sports.
• S0, non-approved substances — “Any pharmacological substance which is not addressed by any of the subsequent sections of the List and with no current approval by any governmental regulatory health authority for human therapeutic use” is prohibited at all times. The 2026 edition names BPC-157 in this class; the compound-level position is separate.
• Specified Substance — a designation under Article 4.2.2 of the Code. All S0 substances are Specified; all S2 substances are not. The comment to Article 4.2.2 of the Code, reproduced in the List’s introduction, states that Specified Substances “should not in any way be considered less important or less dangerous than other doping substances or methods”, only that they are “more likely to have been consumed or used by an Athlete for a purpose other than the enhancement of sport performance”.
• Controlled drug — a substance listed in Schedule 2 to the Misuse of Drugs Act 1971, and placed in one of Schedules 1 to 5 to the Misuse of Drugs Regulations 2001 for the purposes of lawful possession, supply and record-keeping. The status derives from the Act; the 2001 Regulations schedule substances that already hold it. Somatropin is named in Part II of Schedule 4, alongside chorionic gonadotrophin and a list of anabolic agents.
• GB CLP — the classification, labelling and packaging framework applying in Great Britain, assimilated from Regulation (EC) No 1272/2008, with the Health and Safety Executive as the GB CLP Agency. It supplies a laboratory label’s hazard pictograms — a red-bordered diamond on white — and its standard hazard statements.
• Evidence grade — the A to E band on every library entry. A property of the published literature on a substance, not of the substance and not of the product. The criteria are published in full.
• Evidence gap — a named absence rather than a negative finding: no completed randomised controlled trial, no toxicology to regulatory standard, no data in a given model. Three compounds where the gaps dominate are covered together.

References

1. IUPAC-IUB Joint Commission on Biochemical Nomenclature. Nomenclature and symbolism for amino acids and peptides. Recommendations 1983. Eur J Biochem. 1984;138(1):9–37. PMID 6692818. DOI 10.1111/j.1432-1033.1984.tb07877.x. Standards recommendation.
2. World Anti-Doping Agency. World Anti-Doping Code International Standard: Prohibited List 2026, valid 1 January 2026. Introduction (“Specified and non-Specified”, quoting the comment to Code Article 4.2.2) and sections S0, S2.2, S2.3. Regulatory.
3. Lee C, Zeng J, Drew BG, et al. The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance. Cell Metab. 2015;21(3):443–54. PMID 25738459. DOI 10.1016/j.cmet.2015.02.009. Rodent and cell models.
4. Carpenter JF, Pikal MJ, Chang BS, Randolph TW. Rational design of stable lyophilized protein formulations: some practical advice. Pharm Res. 1997;14(8):969–75. PMID 9279875. DOI 10.1023/a:1012180707283. Review.
5. Johnson RE, Kirchhoff CF, Gaud HT. Mannitol-sucrose mixtures — versatile formulations for protein lyophilization. J Pharm Sci. 2002;91(4):914–22. PMID 11948529. DOI 10.1002/jps.10094. Laboratory formulation study.
6. Roux S, Zékri E, Rousseau B, Paternostre M, Cintrat JC, Fay N. Elimination and exchange of trifluoroacetate counter-ion from cationic peptides: a critical evaluation of different approaches. J Pept Sci. 2008;14(3):354–9. PMID 18035848. DOI 10.1002/psc.951. Laboratory analytical study.
7. Murray KK, Boyd RK, Eberlin MN, Langley GJ, Li L, Naito Y. Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013). Pure Appl Chem. 2013;85(7):1515–1609. DOI 10.1351/PAC-REC-06-04-06. Standards recommendation.
8. Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Stability of protein pharmaceuticals: an update. Pharm Res. 2010;27(4):544–75. PMID 20143256. DOI 10.1007/s11095-009-0045-6. Review.
9. US Food and Drug Administration. Guidance for Industry: Pyrogen and Endotoxins Testing — Questions and Answers (Edition 2). March 2026, superseding the June 2012 first edition. Regulatory.
10. Medicines and Healthcare products Regulatory Agency. MHRA Guidance Note 8: A guide to what is a medicinal product. Published September 2025. Regulatory.
11. The Human Medicines Regulations 2012, SI 2012/1916, regulations 2 and 8. In-force consolidated text, not the as-made version. Regulatory.
12. The Misuse of Drugs Act 1971, section 2 and Schedule 2; and The Misuse of Drugs Regulations 2001, SI 2001/3998, Schedule 4 Part II. In-force consolidated text, Schedule 4 as amended to 15 January 2025, not the as-made version. Regulatory.
13. Health and Safety Executive. GB CLP Regulation and Hazard symbols and hazard pictograms. Regulatory.